1. Field of the Invention
The present invention relates to an electrophotographic image forming technique for printers, copier, facsimiles, etc., and more particularly to an electrophotographic image forming technique using wet-type development as a development method and a toner amount detection sensor.
2. Description of the Related Art
In electrophotographic image forming apparatuses, a toner image on a photoconductor is developed by toner using a development device. For example, an electrostatic latent image developed on the photoconductor is then transferred onto recording paper to form an image. In such a transfer process of the image forming apparatus, an electrostatic transfer method is generally adopted.
When a toner image is transferred onto a sheet of paper that is a transfer destination, voltage is applied, for example, by a transfer roller from the back surface of paper arranged to be opposed to the photoconductor, so that an electric field is formed between the photoconductor and the recording paper. The electric field causes the toner image to electrostatically adsorb on the recording paper.
A fixing device then fixes the transferred toner image on the recording paper by pressing and heating the toner image.
In recent years, wet-type development devices are known among image forming apparatuses such as office printers for bulk print and on-demand printers that require higher image quality and higher resolution. The wet-type development devices use a liquid developer that has a small toner particle size and is less likely to cause variations in toner images. The wet-type development devices are advantageous in that high-resolution images are obtained because of the toner mean particle size as small as 0.1 to 2 μm, and that uniform images are obtained because of high flowability of liquid.
In the wet-type image forming apparatus, image quality such as image density can be adjusted by changing image forming conditions including various factors such as a bias voltage applied to each unit of the apparatus. The image density of toner images may vary due to individual differences of apparatuses, changes over time, and changes in environment surrounding the apparatus such as temperature and humidity.
In this respect, a density control technique has been proposed which controls an image density by adjusting an image forming condition that affects image density, among the factors as described above.
For example, Japanese Laid-Open Patent Publication No. 2004-157180 proposes a technique in which a patch image for test is formed on a surface of an image carrier, light is applied to the patch image, light from the patch image is received to detect an image density, and image forming conditions such as a surface potential of a photoconductor and a toner density of a developer are controlled based on the detection result.
In the case of color development, the optimum wavelength of light for detecting an image density varies among colors. Japanese Laid-Open Patent Publication No. 3-111743 discloses a density detection device configured such that a light-emitting device corresponding to each color is provided. Japanese Laid-Open Patent Publication No. 6-27823 proposes a densitometer in which light of a wavelength absorbed in a pigment is emitted.
FIG. 17 shows the result of sensing a toner amount by applying light of a wavelength absorbed in a pigment of each of cyan and yellow developers.
Referring to FIG. 17, the cyan developer is a developer in which toner particles including cyan pigments are dispersed in a carrier liquid.
Here, a red LED is used for the cyan developer. The red LED emits light of a wavelength around 632 nm, where the wavelength of 632 nm is the peak of emission intensity. Light of a wavelength around 632 nm is red light with high absorbance with a cyan pigment.
The horizontal axis represents the toner amount of toner particles included in the developer on an image carrier, and the vertical axis represents a sensor output that is output from a photodiode for use in a light-receiving unit when a developer layer with different toner amounts image density) is detected.
Here, in the figure, the region shown by the dashed lines is a region of a desired toner amount to be detected.
The desired toner amount region includes a target toner amount (toner amount per predetermined area) a on the photoconductor and a toner amount permissible range in the vicinity of the target toner amount.
In order to control the image forming condition based on a sensor output from the toner amount detection sensor, the toner amount detection sensor need to have detection sensitivity in the toner amount permissible range on the image carrier and the toner amount region in the vicinity thereof with the target toner amount a at the center, that is, in the desired toner amount region, and incorporate a difference in toner amount of the developer layer into a difference of the sensor output. It is thus requested that the detection sensitivity should be high in the desired toner amount region.
Referring to the detection result of the toner amount of the cyan developer, the change of the sensor output with respect to the toner amount is great in the desired toner amount region. It can be understood that high detection sensitivity is obtained in the desired toner amount region due to the effect achieved by using an LED of red light with high absorbance with a cyan pigment in the light-emitting unit. In other words, adjustment to the desired toner amount region can be made based on the detection result.
On the other hand, the yellow developer is a developer in which toner particles including yellow pigments are dispersed in a carrier liquid.
Here, a blue LED is used for the yellow developer. The blue LED emits light of a wavelength around 470 nm, where the wavelength of 470 nm is the peak of emission intensity. Light of a wavelength around 470 nm is blue light with high absorbance with a yellow pigment.
Referring to the detection result of the toner amount of the yellow developer, the change of the sensor output with respect to the toner amount is extremely large in a toner amount region smaller than the desired toner amount region. In the desired toner amount region, the sensor output decreases almost to the limit.
Therefore, there is little change in the sensor output with respect to the toner amount in the desired toner amount region. In other words, because of too high detection sensitivity, detection sensitivity cannot be obtained in the desired toner amount region. That is, adjustment to the desired toner amount region is difficult based on the detection result.
Accordingly, when the toner amount detection sensor as described above is used for an image forming apparatus, the sensor cannot output the toner amount on the image carrier accurately in the desired toner amount region, so that it is impossible to properly control an image density (to adjust to the desired toner amount region).
In this respect, the inventor of the present invention conducted a variety of validation experiments about the toner amount detection result of the yellow developer and found that the reason is that the quantity of light received by the light-receiving unit is smaller than expected due to the effects on light given by pigments, specifically, due to the effects of Rayleigh scattering and excessive absorption by pigments.